1. Field of the Invention
The present invention relates to a method for driving a display device, especially a hold-type display device such as a liquid crystal display device, a display device which is driven by the driving method, and a television receiver having the display device.
2. Description of the Related Art
In recent years, instead of a cold cathode ray tube (hereinafter, referred to as CRT), a display device which performs display by electro-optical conversion such as a liquid crystal display (hereinafter, referred to as LCD) panel is widely used in various types of electric and electronic equipment including a television receiver, exploiting its characteristics such as thinness and lower power consumption.
Generally in the above-described display device, image signals for one screen (frame) are inputted at established intervals (e.g., at 60 Hz in the case of television transmission), and accordingly images corresponding to the image signals are displayed one after another. The image of one frame is displayed such that pixels arranged in a matrix in a display screen are successively selected and gray levels in accordance with the image to be displayed on the display screen are written in the pixels.
In a so-called “impulse-type” display device typified by a CRT, pixels which are successively selected in a frame Fn are arranged such that when gray levels in accordance with an image signal for the frame Fn are written in the pixels, fluorescence substances emit light at brightness levels corresponding to the gray levels only for a very short time, and then the brightness levels decay until gray levels for a next frame F(n+1) are written in the pixels as shown in FIG. 13A.
Meanwhile, generally in a so-called “hold-type” display device typified by an LCD panel, pixels which are successively selected in a frame Fn are arranged such that when gray levels in accordance with an image signal for the frame Fn are written in the pixels, the pixels perform display at brightness levels corresponding to the gray levels and the display is maintained in this state until gray levels for a next frame F(n+1) are written in the pixels. In the pixels in this type of display device, the display at the brightness levels corresponding to the gray levels for the previous frame Fn is continued until the gray levels for the next frame F(n+1) are written as shown in FIG. 13B.
FIGS. 14A and 14B are views schematically showing changes in display state in the case of displaying moving images on screens of the above-described display devices of different type. Shown in FIG. 14A are the changes in display state for every 1/240 second in displaying an image of 60 frames/second in the impulse-type display device, and shown in FIG. 14B are the changes in display state for every 1/240 second in displaying an image of 60 frames/second in the hold-type display device.
In the impulse-type display device, an operation such that the image of each frame is displayed and disappears momentarily (i.e., the screen is displayed in black) is repeated successively. For example, as shown in FIG. 14A, in the frame Fn that is an nth frame, a black square 92 is displayed at a position Xn against a white background on a display screen 90 and then nothing is displayed there (i.e., the screen is displayed in black). Then, in the next frame F(n+1) that is an (n+1)th frame, the black square 92 is displayed at a position X(n+1) against the white background. This creates an illusion of smooth moving of the black square 92 from the position Xn to the position X(n+1) taking 1/60 second for a viewer of the display screen 90. Thus, in the impulse-type display device, there is an advantage that the moving image is perceived to be smoothly moving as described above; however, there is a problem that flicker occurs in the display screen 90 to easily cause eyestrain to the viewer.
Meanwhile, in the hold-type display device such as an LCD panel, as shown in FIG. 14B, a black square 96 is displayed at a position Xn against a white background on a display screen 94 in the frame Fn that is an nth frame, is continued to be displayed at the position Xn until just before the frame is advanced to the next frame F(n+1) that is an (n+1)th frame, and is then displayed at a position X(n+1) against the white background in the next frame F(n+1). Thus, in the hold-type display device, there is no problem that flicker occurs in the display screen 94 because there is no period during which no image is displayed (i.e., there is no period during which the screen is displayed in black); however, there is a problem that the moving image suffers unnatural movement since the image of the previous frame is displayed until just before the image of the next frame is displayed and accordingly the frame is momentarily advanced from the previous frame to the next frame.
In order to solve the above-described problems, there is a method of alternately repeating a period during which a gray scale voltage in accordance with an image signal is applied to each pixel electrode in an LCD panel and a period during which a gray scale voltage corresponding to a black level is applied thereto (see, Japanese Patent Application Unexamined Publication No. Hei11-109921). In addition, there is a method of repeating in one frame period an operation of turning off a backlight to display in black during a period of writing a gray scale voltage in each pixel in an LCD panel and turning on the backlight during the other periods (see, Japanese Patent Application Unexamined Publication No. 2000-293142).
However, performing such pseudo impulse-type display by the hold-type display device and providing in each frame period the period during which no image is displayed (i.e., the screen is displayed in black) as described above causes not only a problem of losing the advantage of the hold-type display device such that no flicker occurs in the screen but also a problem of decreasing the brightness levels resulting from a short image display time.